Micro-mirror array devices are devices comprising a plurality of microscopically small mirrors arranged in an array. Such devices comprise micro-electromechanical systems (MEMS) devices whose states are controlled by a voltage between electrodes located around the array.
Micro-mirror array devices are operated to tilt along a certain axis (or axes) in order to deflect incident light. Typically, the tilt of the micro-mirror is controlled by the actuation of electrodes associated with the micro-mirror, for example, by using an applied voltage.
Characterisation of voltage against tilt angle for a given micro-mirror device is important in evaluating its performance. Furthermore, this relationship of voltage against tilt angle is also important in calibrating a micro-mirror for use in a certain application, for example, in “smart” lenses where micro-mirrors are used in variable focal length lenses to make zoom lenses. In addition, obtaining or characterising the voltage-tilt angle relationship at run-time is often desirable to support run-time calibration.
Current methods used for characterising voltage against tilt angle of a micro-mirror typically comprise optical metrology techniques. In such techniques, the micro-mirror is inspected using microscopes of some sort. These methods are quite sensitive, reasonably fast and suitable in many cases where characterisation has to be performed only once before the device is put to use.
These optical metrology techniques are not suitable in applications where run-time characterisation is required, for example, where the micro-mirror array device is configured as a variable focal length lens. This is because such metrology systems tend to be time consuming, for example, in order to characterise every micro-mirror element in a micro-mirror array comprising 350,000 micro-mirror elements may require more than 30 minutes. This limits the production throughput, that is, the number of micro-mirror arrays that can be produced in a given period of time.
In addition, optical metrology methods cannot be employed for online or run-time calibration. If the voltage-tilt angle characteristic drifts over a period of operation of a micro-mirror array device, it is not possible to take into account that drift as re-characterisation of voltage-tilt angle characteristic cannot be performed once the micro-mirror array device is operational.